Configuring Cisco Fax Relay

This chapter describes configuration for Cisco fax relay on an IP network. With Cisco fax relay, gateways terminate T.30 fax signaling by spoofing a virtual fax machine to the locally attached fax machine. The gateways use a Cisco-proprietary fax-relay RTP-based protocol to communicate between them.

History for the Cisco Fax Relay Feature

Release

Modification

12.2(11)T

This feature was introduced.

12.4(4)T

The fax-relaysg3-to-g3 command was integrated into Cisco IOS release 12.4(4)T

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn . You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Note

For more information about this and related Cisco IOS voice features, see the Cisco IOS Voice Configuration Library ; including library preface and glossary, other feature documents, and troubleshooting documentation.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

If you use modem pass-through to send SG3 faxes and you use Cisco fax relay to send G3 faxes, you must configure both modem pass-through and fax relay.

When a two-gateway solution is used, both gateways must be configured to use SG3 V.8 fax CM message suppression.

When a one-gateway solution is used, other gateways can be Cisco gateways that do not support SG3 V.8 fax CM message suppression or third-party gateways that are not SG3-capable if the fax CM message suppression gateway is the originating gateway.

SG3 fax machines will scale down to G3 speeds if the SG3 V.8 fax CM message is suppressed or if the signals are not delivered reliably by low bit rate codecs.

Information About Cisco Fax Relay

Methods for Fax Relay

Cisco provides two methods for fax relay. One method is a Cisco-proprietary method called Cisco fax relay, and it is described in this chapter. The second method is based on the ITU-T T.38 standard, and it is described in "Configuring T.38 Fax Relay."

T.38 fax relay is the default mode for passing faxes through a VoIP network, and Cisco fax relay is the default fax relay type on Cisco voice gateways. This capability has been supported in Cisco IOS Release 11.3 and later releases and is widely available. Cisco fax relay uses Real-Time Transport Protocol (RTP) to transport the fax data.

Cisco fax relay is configured on the VoIP dial peers that direct calls into and out of the packet network.Cisco fax relay can be configured under the H.323 and Session Initiation Protocol (SIP) call control protocols.

Fax Relay Packet Loss Concealment

Cisco fax relay supports fax relay packet loss concealment, which is a technique that allows gateways to disregard packet loss rates that might otherwise cause fax failures. High-end fax machines with the memory to store page data often are able to use Error Correction Mode (ECM) for error-free page transmission. When ECM is enabled, a fax page is transmitted in a series of blocks that contain frames with packets of data. After receiving the data for a complete page, a receiving fax machine notifies the transmitting fax machine of any frames with errors. The transmitting fax machine then retransmits the specified frames. This process is repeated until all frames are received without errors. If the receiving fax machine is unable to receive an error-free page, the fax transmission may fail and one of the fax machines may disconnect. On networks that have a packet loss rate greater than 2 per cent, fax transmissions routinely fail when ECM is enabled because of ECM's low tolerance for packet loss.

The Fax Relay Packet Loss Concealment feature allows you to control whether ECM is enabled or disabled for fax transmissions on a VoIP dial peer. By disabling ECM on networks with a large amount of packet loss, you ensure that more fax transmissions are completed, although they may not be totally error-free.

When ECM is disabled, a fax page is transmitted using high-speed modulation in its raw encoded format. When detecting line errors with ECM disabled, the receiving fax machine has three options (in order of increasing severity):

Respond to page reception with the ReTrain Positive command. This response causes the transmitting fax to go through the training check process before transmitting the next page.

Respond to the page reception with the ReTrain Negative command. This response causes the transmitting fax to go through the Training Check Frame (TCF) process with a lower modulation scheme.

Disconnect immediately.

Fax relay ECM is enabled by default. To disable ECM, you use the fax-relayecmdisable command on the VoIP dial peer. After this command is configured, the gateway's Digital Signal Processor (DSP) fax-relay firmware modifies the T.30 Digital Information Signal (DIS) message. This modification is performed on DIS signals in both directions, so that ECM is disabled even when only one gateway is configured to disable ECM.

Disabling of ECM is recommended for dial peers handling fax relay traffic on known lossy networks, especially those with a packet loss rate of 2 percent or greater. The debugfaxrelayt30 command provides information about the E.164 destination and T.30 messages associated with fax transmissions. Note than an excessive number of simultaneous debug operations can degrade performance.

Fax CM Message Tone Suppression

Super Group 3 (SG3) is a new generation of fax machines that support speeds of up to 33.6 kbps through V.34 half duplex (HD) modulation and V.8 signaling.

SG3 V.8 fax CM message tone suppression enables SG3 fax machines to scale down without end-user interaction and without using the extra bandwidth required by modem pass-through and allows SG3 fax machines to interoperate over a fax-relay network at G3 speeds by blocking the SG3 V.8 CM message, or fax tone, from reaching the called fax machine. This causes the called fax machine to time out on the ANSam tone and scale down to G3 speeds by initiating V.21 negotiations.

With a one-gateway solution, the gateway on one end of the call can be configured to suppress the SG3 V.8 fax CM message independently of the gateway on the other end of the call. The one-gateway solution suppresses the fax CM tone on both TDM and IP interfaces (TI C5510 DSPs only), and can interoperate with third-party gateways when the fax CM tone suppression gateway is the originating gateway. A one-gateway solution

With a two-gateway solution, the gateways on both ends of the call must have this feature enabled. The two-gateway solution suppresses the fax CM tone only on the TDM interface (TI C5421 and TI C549 DSPs). Both gateways must support this feature to interoperate at G3 speeds, or the fax tone suppression gateway must be the originating gateway.

Note

If both the originating gateway and the terminating gateways are configured for V.8 fax CM message suppression, the suppression occurs on the originating gateway.

How to Configure Cisco Fax Relay

Cisco fax relay can be configured globally for all VoIP dial peers or for individual dial peers. This section contains the following tasks:

Note

Fax relay parameters that are set for an individual dial peer under the dial-peervoice command take precedence over global settings made under the voiceservicevoip command.

(Optional) Selects the fax transmission speed to be attempted when this dial peer is used.

12000, 14400, 2400, 4800, 7200, 9600--Maximum bits-per-second speed.

disable--Disables fax relay transmission capability.

voice--Highest possible transmission speed allowed by the voice rate. For example, if the voice codec is G.711, fax transmission occurs at up to 14400 bps because 14400 bps is less than the 64-kbps voice rate. If the voice codec is G.729 (8 kbps), the fax transmission speed is 7200 bps. This is the default.

bytesrate--(Optional) Fax packetization rate, in ms. Range is 20 to 48. The default is 20.

Step 6

fax-relayecmdisable

Example:

Router(config-dial-peer)# fax-relay ecm disable

(Optional) Disables fax-relay ECM.

Note

To enable ECM, use the no form of this command.

Step 7

faxnsfword

Example:

Router(config-dial-peer)# fax nsf 000000

(Optional) Allows the router to override the settings made by fax machines that try to implement proprietary encodings (non-standard facilities, or NSF). By default, the NSF code is not overridden.

word--Two-digit hexadecimal country code and a four-digit hexadecimal manufacturer code.

Note

Setting this command to all zeroes prevents transfer of NSF during fax negotiation and overwrites the NSF so that only standard fax transactions occur. Because a router demodulates and decodes fax tones based on the T.30 specification, transactions or encoding that are proprietary can cause fax relay transmissions to fail.

Step 8

fax-relaysg3-to-g3system

Example:

Router(config-dial-peer)# fax-relay sg3-to-g3 system

Specifies that for SIP and H.323 signaling types, V.8 fax CM message suppression is enabled on the specific dial peer. Enabled by default.

system--Uses the protocol set under the voice-service configuration.

Configuring Cisco Fax Relay for VoIP Dial Peers Globally

Use the following steps to configure Cisco fax relay globally for VoIP dial peers.

Note

Fax relay parameters that are set for an individual dial peer under the dial-peervoice command take precedence over global settings made under the voiceservicevoip command.

SUMMARY STEPS

1.enable

2.configureterminal

3.voiceservicevoip

4.faxprotocol {cisco | none}

5.fax-relaysg3-to-g3

DETAILED STEPS

Command or Action

Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2

configureterminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

voiceservicevoip

Example:

Router(config)# voice service voip

Enters voice-service configuration mode.

Step 4

faxprotocol {cisco | none}

Example:

Router(config-voi-serv)# fax protocol cisco

Specifies the fax protocol for all dial peers.

cisco--Cisco-proprietary fax protocol.

none--Disables fax relay and fax pass-through.

Step 5

fax-relaysg3-to-g3

Example:

Router(config-voi-serv)# fax-relay sg3-to-g3

(Optional) Specifies that for SIP and H.323 signaling types, V.8 fax CM message suppression is enabled for all dial peers on the digital signal processor (DSP) firmware. Enabled by default.

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